I know a photo is worth a thousand words but I'll have to rely on words.

I found on the underside of a large concrete bridge what appeared to be some kind of egg sacks or webs. There were hundreds. They were oval shaped, white and from 1/2 to 3/4 inch long. The material was silk-like They were slightly raised (convex). I checked one but could not make out what it was. The underside of the bridge was covered with them and the remains of what were previous generations.

The bridge was over a narrow muddy but moderately fast moving river in southwest Tennessee, the Wolf River

Any ideas?